Potential Sources of Resistance for Melon to Nonpersistently Aphid-borne Viruses

Comprehensive proteomic profiling of Cucumber mosaic virus infection: identifying key proteins and pathways involved in resistance and susceptibility in melon

BACKGROUND

Melon (Cucumis melo L.) is the model species of the Cucurbitaceae family and an important crop. However, its yield is primarily affected by viruses. Cucumber mosaic virus (CMV) is particularly significant due to its broad host range, capable of infecting over 100 plant families. Resistance to CMV in the melon accession Songwhan Charmi (SC) is controlled by the recessive gene cmv1, which encodes the Vacuolar Protein Sorting 41, involved in vesicle transport to the vacuole. cmv1 restricts the virus to the bundle sheath cells and impedes viral access to the phloem, preventing a systemic infection. This phenotype depends on the viral movement protein (MP). However, little is known about the broader cellular changes that CMV triggers in melon or the specific biological responses that facilitate or restrict the virus entry into the phloem in susceptible and resistant varieties.

RESULT

We profiled the proteomes of CMV-resistant or susceptible melon genotypes inoculated with CMV-LS or FNY strains. Analysis of co-abundance networks revealed the rewiring of central biological pathways during different stages of CMV infection. Upon inoculation, resistant varieties do not trigger any signalling event to the new leaves. Local infection triggers a general depletion in proteins related to translation, photosynthesis and intracellular transport, whereas only in resistant varieties CMV triggers an increase in lipid modification and phloem proteins. During the systemic infection of susceptible melon plants, there is a strong increase in proteins associated with stress responses, such as those involved in the ER-associated degradation (ERAD) and phenylpropanoid pathways, along with a decrease in translation and photosynthesis. Key hub proteins have been identified in these processes.

CONCLUSIONS

This study is the first comprehensive high-throughput proteomic analysis of CMV-infected melon plants, providing a novel and detailed understanding of the proteomic changes associated with CMV infection, highlighting the differential responses between resistant and susceptible genotypes and identifying key proteins that could be potential targets for future research and CMV management strategies.

Aphis gossypii (Hemiptera: Aphididae) feeding responses to double virus infections in melon

Virus infections cause economic losses in crops worldwide and their management and control present major challenges. In the field, double infections of two or more viruses are the rule, not the exception. The presence of several viruses in a plant makes it difficult to interpret virus-insect vector-plant interactions. Mixed infections can alter plant symptoms compared with single infections and may also impact their vectors. We describe plant-mediated indirect effects of virus double-infection on feeding behavior of an aphid vector (Aphis gossypii Glover) and virus transmission in melon (Cucumis melo L.). The viruses we used were a circulative cucurbit aphid-borne yellows virus (CABYV, Solemoviridae:Polerovirus) and a non-circulative cucumber mosaic virus (CMV, Bromoviridae:Cucumovirus). When melon plants were CMV-infected, indirect plant-mediated effects on A. gossypii feeding were like those reported on other plant species; specifically, intracellular punctures (pd) were more frequent and longer than on mock-inoculated plants, which enhanced CMV transmission. Similarly, when plants were CABYV-infected, we observed a statistically non-significant trend for increases in extended salivation (E1) and ingestion (E2) activities in phloem compared with mock-inoculated plants, which also enhanced CABYV transmission. When aphids fed on CMV-CABYV double-infected plants feeding behavior activities related to the transmission of both viruses were enhanced even more than when feeding on single-infected plants. Nevertheless, the virus transmission rate was the same on single-infected or double-infected plants. Thus, our results suggest that double infections do not modify viral dispersion compared with single infections since the indirect effect of CMV and CABYV in single infections is already optimized to favor virus transmission.

Possibility of genome editing for melon breeding

Genome editing technologies are promising for conventional mutagenesis breeding, which takes a long time to remove unnecessary mutations through backcrossing and create new lines because they directly modify the target genes of elite strains. In particular, this technology has advantages for traits caused by the loss of function. Many efforts have been made to utilize this technique to introduce valuable features into crops, including maize, soybeans, and tomatoes. Several genome-edited crops have already been commercialized in the US and Japan. Melons are an important vegetable crop worldwide, produced and used in various areas. Therefore, many breeding efforts have been made to improve its fruit quality, resistance to plant diseases, and stress tolerance. Quantitative trait loci (QTL) analysis was performed, and various genes related to important traits were identified. Recently, several studies have shown that the CRISPR/Cas9 system can be applied to melons, resulting in its possible utilization as a breeding technique. Focusing on two productivity-related traits, disease resistance, and fruit quality, this review introduces the progress in genetics, examples of melon breeding through genome editing, improvements required for breeding applications, and the possibilities of genome editing in melon breeding.

Insights into the early transcriptomic response against watermelon mosaic virus in melon

BACKGROUND

Watermelon mosaic virus (WMV) is one of the most prevalent viruses affecting melon worldwide. Recessive resistance to WMV in melon has previously been reported in the African accession TGR-1551. Moreover, the genomic regions associated to the resistance have also been described. Nevertheless, the transcriptomic response that might infer the resistance to this potyvirus has not been explored.

RESULTS

We have performed a comparative transcriptomic analysis using mock and WMV-inoculated plants of the susceptible cultivar "Bola de oro" (BO) and a resistant RIL (Recombinant inbred line) derived from the initial cross between "TGR-1551" and BO. In total, 616 genes were identified as differentially expressed and the weighted gene co-expression network analysis (WGCNA) detected 19 gene clusters (GCs), of which 7 were differentially expressed for the genotype x treatment interaction term. SNPs with a predicted high impact on the protein function were detected within the coding regions of most of the detected DEGs. Moreover, 3 and 16 DEGs were detected within the QTL regions previously described in chromosomes 11 and 5, respectively. In addition to these two specific genomic regions, we also observde large transcriptomic changes from genes spread across the genome in the resistant plants in response to the virus infection. This early response against WMV implied genes involved in plant-pathogen interaction, plant hormone signal transduction, the MAPK signaling pathway or ubiquitin mediated proteolysis, in detriment to the photosynthetic and basal metabolites pathways. Moreover, the gene MELO3C021395, which coded a mediator of RNA polymerase II transcription subunit 33A (MED33A), has been proposed as the candidate gene located on chromosome 11 conferring resistance to WMV.

CONCLUSIONS

The comparative transcriptomic analysis presented here showed that, even though the resistance to WMV in TGR-1551 has a recessive nature, it triggers an active defense response at a transcriptomic level, which involves broad-spectrum resistance mechanisms. Thus, this study represents a step forward on our understanding of the mechanisms underlaying WMV resistance in melon. In addition, it sheds light into a broader topic on the mechanisms of recessive resistances.

Occurrence, Distribution, and Management of Aphid-Transmitted Viruses in Cucurbits in Spain

The effectiveness of pest and disease management in crops relies on knowledge about their presence and distribution in crop-producing areas. Aphids and whiteflies are among the main threats to vegetable crops since these hemipterans feed on plants, causing severe damage, and are also able to transmit a large number of devastating plant viral diseases. In particular, the widespread occurrence of aphid-transmitted viruses in cucurbit crops, along with the lack of effective control measures, makes surveillance programs and virus epidemiology necessary for providing sound advice and further integration into the management strategies that can ensure sustainable food production. This review describes the current presence and distribution of aphid-transmitted viruses in cucurbits in Spain, providing valuable epidemiological information, including symptom expressions of virus-infected plants for further surveillance and viral detection. We also provide an overview of the current measures for virus infection prevention and control strategies in cucurbits and indicate the need for further research and innovative strategies against aphid pests and their associated viral diseases.

Resistance to Cucumber Green Mottle Mosaic Virus in Cucumis melo

Cucumber green mottle mosaic virus (CGMMV) is a severe threat to melon production worldwide. At present, there are no cultivars available on the market which show an effective resistance or tolerance to CGMMV infection; only wild Cucumis species were reported as resistant. Germplasm accessions of Cucumis melo, as well as C. anguria, C. ficifolius, C. myriocarpus and C. metuliferus, were mechanically infected with isolates belonging to the European and Asian strain of CGMMV and screened for resistance by scoring symptom severity and comparing the accumulation of virus by qRT-PCR. The wild species C. anguria and C. ficifolius showed no symptoms and did not accumulate CGGMV following inoculation, while C. metuliferus was highly susceptible to the isolates of both strains of CGMMV. The virus accumulated also in C. myriocarpus and the European isolate produced symptoms, but the Asian isolate did not. Thirty C. melo accessions were susceptible to CGMMV. An isolate-dependent expression of symptoms was observed in 16 melon accessions: they showed mild and severe symptoms at 14 and 21 days after inoculation with the European and Asian isolate, respectively. Freeman's Cucumber showed few or no symptoms following inoculation with the isolate of either CGMMV strain. This particular accession also showed reduced virus accumulation, whereas most other tested germplasm accessions showed significantly higher viral loads and, therefore, may well be a candidate for breeding programs aiming to reduce the losses produced by CGMMV with resistant commercial melon cultivars.

CmVPS41 Is a General Gatekeeper for Resistance to Cucumber Mosaic Virus Phloem Entry in Melon

Melon production is often compromised by viral diseases, which cannot be treated with chemicals. Therefore, the use of genetic resistances is the main strategy for generating crops resistant to viruses. Resistance to Cucumber mosaic virus (CMV) in melon is scarcely described in few accessions. Until recently, the only known resistant accessions were Freeman's Cucumber and PI 161375, cultivar Songwhan Charmi (SC). Resistance to CMV in melon is recessive and generally oligogenic and quantitative. However, in SC, the resistance to CMV strains of subgroup II is monogenic, depending only on one gene, cmv1, which is able to stop CMV movement by restricting the virus to the bundle sheath cells and preventing a systemic infection. This restriction depends on the viral movement protein (MP). Chimeric viruses carrying the MP of subgroup II strains, like the strain LS (CMV-LS), are restricted in the bundle sheath cells, whereas those carrying MP from subgroup I, like the strain FNY (CMV-FNY), are able to overcome this restriction. cmv1 encodes a vacuolar protein sorting 41 (CmVPS41), a protein involved in the transport of cargo proteins from the Golgi to the vacuole through late endosomes. We have analyzed the variability of the gene CmVPS41 in a set of 52 melon accessions belonging to 15 melon groups, both from the spp melo and the spp agrestis. We have identified 16 different haplotypes, encoding 12 different CmVPS41 protein variants. Challenging members of all haplotypes with CMV-LS, we have identified nine new resistant accessions. The resistance correlates with the presence of two mutations, either L348R, previously found in the accession SC and present in other three melon genotypes, or G85E, present in Freeman's Cucumber and found also in four additional melon genotypes. Moreover, the new resistant accessions belong to three different melon horticultural groups, Conomon, Makuwa, and Dudaim. In the new resistant accessions, the virus was able to replicate and move cell to cell, but was not able to reach the phloem. Therefore, resistance to phloem entry seems to be a general strategy in melon controlled by CmVPS41. Finally, the newly reported resistant accessions broaden the possibilities for the use of genetic resistances in new melon breeding strategies.

A mutation in the melon Vacuolar Protein Sorting 41prevents systemic infection of Cucumber mosaic virus

In the melon exotic accession PI 161375, the gene cmv1, confers recessive resistance to Cucumber mosaic virus (CMV) strains of subgroup II. cmv1 prevents the systemic infection by restricting the virus to the bundle sheath cells and impeding viral loading to the phloem. Here we report the fine mapping and cloning of cmv1. Screening of an F2 population reduced the cmv1 region to a 132 Kb interval that includes a Vacuolar Protein Sorting 41 gene. CmVPS41 is conserved among plants, animals and yeast and is required for post-Golgi vesicle trafficking towards the vacuole. We have validated CmVPS41 as the gene responsible for the resistance, both by generating CMV susceptible transgenic melon plants, expressing the susceptible allele in the resistant cultivar and by characterizing CmVPS41 TILLING mutants with reduced susceptibility to CMV. Finally, a core collection of 52 melon accessions allowed us to identify a single amino acid substitution (L348R) as the only polymorphism associated with the resistant phenotype. CmVPS41 is the first natural recessive resistance gene found to be involved in viral transport and its cellular function suggests that CMV might use CmVPS41 for its own transport towards the phloem.

Four sequence positions of the movement protein of Cucumber mosaic virus determine the virulence against cmv1-mediated resistance in melon

The resistance to a set of strains of Cucumber mosaic virus (CMV) in the melon accession PI 161375, cultivar 'Songwhan Charmi', is dependent on one recessive gene, cmv1, which confers total resistance, whereas a second set of strains is able to overcome it. We tested 11 strains of CMV subgroups I and II in the melon line SC12-1-99, which carries the gene cmv1, and showed that this gene confers resistance to strains of subgroup II only and that restriction is not related to either viral replication or cell-to-cell movement. This is the first time that a resistant trait has been correlated with CMV subgroups. Using infectious clones of the CMV strains LS (subgroup II) and FNY (subgroup I), we generated rearrangements and viral chimaeras between both strains and established that the determinant of virulence against the gene cmv1 resides in the first 209 amino acids of the movement protein, as this region from FNY is sufficient to confer virulence to the LS clone in the line SC12-1-99. A comparison of the sequences of the strains of both subgroups in this region shows that there are five main positions shared by all strains of subgroup II, which are different from those of subgroup I. Site-directed mutagenesis of the CMV-LS clone to substitute these residues for those of CMV-FNY revealed that a combination of four of these changes [the group 64-68 (SNNLL to HGRIA), and the point mutations R81C, G171T and A195I] was required for a complete gain of function of the LS MP in the resistant melon plant.

Combined use of genetic and genomics resources to understand virus resistance and fruit quality traits in melon

The availability of the genome sequence of many crop species during the past few years has opened a new era in plant biology, allowing for the performance of massive genomic studies in plant species other than the classical models Arabidopsis and rice. One of these crop species is melon (Cucumis melo), a cucurbit of high economic value that has become an interesting model for the study of biological processes such as fruit ripening, sex determination and phloem transport. The recent availability of the melon genome sequence, together with a number of genetic and genomic resources, provides powerful tools that can be used to assist in the main melon breeding targets, namely disease resistance and fruit quality. In this review, we will describe recent data obtained combining the use of a melon near isogenic line (NIL) population and genomic resources to gain insight into agronomically important traits as fruit ripening, resistance to Cucumber Mosaic virus (CMV) and the accumulation of sugars in fruits.

The effect of Chrysoperla carnea (Neuroptera: Chrysopidae) and Adalia bipunctata (Coleoptera: Coccinellidae) on the spread of cucumber mosaic virus (CMV) by Aphis gossypii (Hemiptera: Aphididae)

The effects of two aphidophagous predators, the larvae of Chrysoperla carnea and adults of Adalia bipunctata, on the spread of cucumber mosaic virus (CMV) transmitted in a non-persistent manner by the cotton aphid Aphis gossypii were studied under semi-field conditions. Natural enemies and aphids were released inside insect-proof cages (1 m × 1 m × 1 m) with a central CMV-infected cucumber plant surrounded by 48 healthy cucumber seedlings, and the spatiotemporal dynamics of the virus and vector were evaluated in the short and long term (1 and 5 days) in the presence and absence of the natural enemy. The spatial analysis by distance indices methodology together with other indices measuring the dispersal around a single focus was used to assess the spatial pattern and the degree of association between the virus and its vector. Both natural enemies significantly reduced the number of aphids in the CMV-source plant after 5 days but not after 1 day. The CMV transmission rate was generally low, especially after 1 day, due to the limited movement of aphids from the central CMV-source plant, which increased slightly after 5 days. Infected plants were mainly located around the central virus-infected source plant, and the percentage of aphid occupation and CMV-infected plants did not differ significantly in absence and presence of natural enemies. The distribution patterns of A. gossypii and CMV were only coincident close to the central plant. The complexity of multitrophic interactions and the role of aphid predators in the spread of CMV are discussed.

A non-persistently transmitted-virus induces a pull-push strategy in its aphid vector to optimize transmission and spread

Plant viruses are known to modify the behaviour of their insect vectors, both directly and indirectly, generally adapting to each type of virus-vector relationship in a way that enhances transmission efficiency. Here, we report results of three different studies showing how a virus transmitted in a non-persistent (NP) manner (Cucumber mosaic virus; CMV, Cucumovirus) can induce changes in its host plant, cucumber (Cucumis sativus cv. Marumba) that modifies the behaviour of its aphid vector (Aphis gossypii Glover; Hemiptera: Aphididae) in a way that enhances virus transmission and spread non-viruliferous aphids changed their alighting, settling and probing behaviour activities over time when exposed to CMV-infected and mock-inoculated cucumber plants. Aphids exhibited no preference to migrate from CMV-infected to mock-inoculated plants at short time intervals (1, 10 and 30 min after release), but showed a clear shift in preference to migrate from CMV-infected to mock-inoculated plants 60 min after release. Our free-choice preference assays showed that A. gossypii alates preferred CMV-infected over mock-inoculated plants at an early stage (30 min), but this behaviour was reverted at a later stage and aphids preferred to settle and reproduce on mock-inoculated plants. The electrical penetration graph (EPG) technique revealed a sharp change in aphid probing behaviour over time when exposed to CMV-infected plants. At the beginning (first 15 min) aphid vectors dramatically increased the number of short superficial probes and intracellular punctures when exposed to CMV-infected plants. At a later stage (second hour of recording) aphids diminished their feeding on CMV-infected plants as indicated by much less time spent in phloem salivation and ingestion (E1 and E2). This particular probing behaviour including an early increase in the number of short superficial probes and intracellular punctures followed by a phloem feeding deterrence is known to enhance the transmission efficiency of viruses transmitted in a NP manner. We conclude that CMV induces specific changes in a plant host that modify the alighting, settling and probing behaviour of its main vector A. gossypii, leading to optimum transmission and spread of the virus. Our findings should be considered when modelling the spread of viruses transmitted in a NP manner.

Spatio-temporal dynamics of viruses are differentially affected by parasitoids depending on the mode of transmission

Relationships between agents in multitrophic systems are complex and very specific. Insect-transmitted plant viruses are completely dependent on the behaviour and distribution patterns of their vectors. The presence of natural enemies may directly affect aphid behaviour and spread of plant viruses, as the escape response of aphids might cause a potential risk for virus dispersal. The spatio-temporal dynamics of Cucumber mosaic virus (CMV) and Cucurbit aphid-borne yellows virus (CABYV), transmitted by Aphis gossypii in a non-persistent and persistent manner, respectively, were evaluated at short and long term in the presence and absence of the aphid parasitoid, Aphidius colemani. SADIE methodology was used to study the distribution patterns of both the virus and its vector, and their degree of association. Results suggested that parasitoids promoted aphid dispersion at short term, which enhanced CMV spread, though consequences of parasitism suggest potential benefits for disease control at long term. Furthermore, A. colemani significantly limited the spread and incidence of the persistent virus CABYV at long term. The impact of aphid parasitoids on the dispersal of plant viruses with different transmission modes is discussed.

Simple sequence repeat markers linked to QTL for resistance to Watermelon mosaic virus in melon

A population of recombinant inbred lines (RIL) derived from a cross between the Watermelon mosaic virus (WMV) resistant genotype TGR-1551 and the susceptible Spanish cultivar 'Bola de Oro' has been evaluated for WMV resistance in spring, fall and growth chamber conditions. The quantitative trait loci (QTL) analyses detected one major QTL (wmv) on linkage group (LG) XI close to the microsatellite marker CMN04_35. This QTL controls the resistance to WMV in the three environmental conditions evaluated. Other minor QTLs affecting the severity of viral symptoms were identified, but they were not detected in all the assayed environments. The screening of the marker CMN04_35 in an F(2) progeny, derived from the same cross, confirmed the effect of this QTL on the expression of WMV resistance also in early generations, which evidences the usefulness of this marker for a marker assisted selection program.

Analysis of the melon (Cucumis melo) small RNAome by high-throughput pyrosequencing

BACKGROUND

Melon (Cucumis melo L.) is a commercially important fruit crop that is cultivated worldwide. The melon research community has recently benefited from the determination of a complete draft genome sequence and the development of associated genomic tools, which have allowed us to focus on small RNAs (sRNAs). These are short, non-coding RNAs 21-24 nucleotides in length with diverse physiological roles. In plants, they regulate gene expression and heterochromatin assembly, and control protection against virus infection. Much remains to be learned about the role of sRNAs in melon.

RESULTS

We constructed 10 sRNA libraries from two stages of developing ovaries, fruits and photosynthetic cotyledons infected with viruses, and carried out high-throughput pyrosequencing. We catalogued and analysed the melon sRNAs, resulting in the identification of 26 known miRNA families (many conserved with other species), the prediction of 84 melon-specific miRNA candidates, the identification of trans-acting siRNAs, and the identification of chloroplast, mitochondrion and transposon-derived sRNAs. In silico analysis revealed more than 400 potential targets for the conserved and novel miRNAs.

CONCLUSION

We have discovered and analysed a large number of conserved and melon-specific sRNAs, including miRNAs and their potential target genes. This provides insight into the composition and function of the melon small RNAome, and paves the way towards an understanding of sRNA-mediated processes that regulate melon fruit development and melon-virus interactions.

Minimising losses caused by Zucchini yellow mosaic virus in vegetable cucurbit crops in tropical, sub-tropical and Mediterranean environments through cultural methods and host resistance

Between 2006 and 2009, 10 field experiments were done at Kununurra, Carnarvon or Medina in Western Australia (WA) which have tropical, sub-tropical and Mediterranean climates, respectively. These experiments investigated the effectiveness of cultural control measures in limiting ZYMV spread in pumpkin, and single-gene resistance in commercial cultivars of pumpkin, zucchini and cucumber. Melon aphids (Aphis gossypii) colonised field experiments at Kununurra; migrant green peach aphids (Myzus persicae) visited but did not colonise at Carnarvon and Medina. Cultural control measures that diminished ZYMV spread in pumpkin included manipulation of planting date to avoid exposing young plants to peak aphid vector populations, deploying tall non-host barriers (millet, Pennisetum glaucum) to protect against incoming aphid vectors and planting upwind of infection sources. Clustering of ZYMV-infected pumpkin plants was greater without a 25m wide non-host barrier between the infection source and the pumpkin plants than when one was present, and downwind compared with upwind of an infection source. Host resistance gene zym was effective against ZYMV isolate Knx-1 from Kununurra in five cultivars of cucumber. In zucchini, host resistance gene Zym delayed spread of infection (partial resistance) in 2 of 14 cultivars but otherwise did not diminish final ZYMV incidence. Zucchini cultivars carrying Zym often developed severe fruit symptoms (8/14), and only the two cultivars in which spread was delayed and one that was tolerant produced sufficiently high marketable yields to be recommended when ZYMV epidemics are anticipated. In three pumpkin cultivars with Zym, this gene was effective against isolate Cvn-1 from Carnarvon under low inoculum pressure, but not against isolate Knx-1 under high inoculum pressure, although symptoms were milder and marketable yields greater in them than in cultivars without Zym. These findings allowed additional cultural control recommendations to be added to the existing Integrated Disease Management strategy for ZYMV in vegetable cucurbits in WA, but necessitated modification of its recommendations over deployment of cultivars with resistance genes.

Dissection of the oligogenic resistance to Cucumber mosaic virus in the melon accession PI 161375

Resistance to Cucumber mosaic virus (CMV) in the exotic melon accession PI 161375, cultivar "Sonwang Charmi" (SC) had previously been described as oligogenic, recessive and quantitative, with a major QTL residing in linkage group XII (LGXII). We have used a collection of near isogenic lines (NILs) with introgressions of SC into the genome of the susceptible accession Piel de Sapo (PS) to further characterise this resistance. Infection of NILs carrying introgressions on LGXII showed that only NIL SC12-1 was resistant to CMV strains P9 and P104.82, but not to strains M6 and TL. Further mapping of this region showed that the resistance, named cmv1 maps in an area of 2.2 cM, between markers CMN61_44 and CMN21_55. Moreover, cmv1 confers total resistance to strains P9 and P104.82, indicating that in these cases it is not quantitative and that cmv1 is sufficient to confer full resistance to these CMV strains. Candidate gene mapping of ten translation initiation factors in the melon genome failed to find any of them in the interval between markers CMN61_44 and CMN21_55. All these results suggest that the resistance to CMV present in SC is oligogenic, where different loci confer resistance to different CMV strains, but not necessarily quantitative, since at least one of these genes (cmv1) confers total resistance, similar to that of the parental SC, and does not need the contribution of other loci.

EcoTILLING for the identification of allelic variants of melon eIF4E, a factor that controls virus susceptibility

BACKGROUND

Translation initiation factors of the 4E and 4G protein families mediate resistance to several RNA plant viruses in the natural diversity of crops. Particularly, a single point mutation in melon eukaryotic translation initiation factor 4E (eIF4E) controls resistance to Melon necrotic spot virus (MNSV) in melon. Identification of allelic variants within natural populations by EcoTILLING has become a rapid genotype discovery method.

RESULTS

A collection of Cucumis spp. was characterised for susceptibility to MNSV and Cucumber vein yellowing virus (CVYV) and used for the implementation of EcoTILLING to identify new allelic variants of eIF4E. A high conservation of eIF4E exonic regions was found, with six polymorphic sites identified out of EcoTILLING 113 accessions. Sequencing of regions surrounding polymorphisms revealed that all of them corresponded to silent nucleotide changes and just one to a non-silent change correlating with MNSV resistance. Except for the MNSV case, no correlation was found between variation of eIF4E and virus resistance, suggesting the implication of different and/or additional genes in previously identified resistance phenotypes. We have also characterized a new allele of eIF4E from Cucumis zeyheri, a wild relative of melon. Functional analyses suggested that this new eIF4E allele might be responsible for resistance to MNSV.

CONCLUSION

This study shows the applicability of EcoTILLING in Cucumis spp., but given the conservation of eIF4E, new candidate genes should probably be considered to identify new sources of resistance to plant viruses. Part of the methodology described here could alternatively be used in TILLING experiments that serve to generate new eIF4E alleles.

Cucurbit aphid-borne yellows virus Is Prevalent in Field-Grown Cucurbit Crops of Southeastern Spain

Despite the importance of field-grown cucurbits in Spain, only limited information is available about the impact of disease on their production. During the 2003 and 2004 growing seasons, systematic surveys were carried out in open field melon (Cucumis melo) and squash (Cucurbita pepo) crops of Murcia Province (Spain). The fields were chosen with no previous information regarding their sanitation status, and samples were taken from plants showing viruslike symptoms. Samples were analyzed using molecular hybridization to detect Beet pseudo-yellows virus (BPYV), Cucurbit aphid-borne yellows virus (CABYV), Cucumber mosaic virus (CMV), Cucumber vein yellowing virus (CVYV), Cucurbit yellow stunting disorder virus (CYSDV), Melon necrotic spot virus (MNSV), Papaya ringspot virus (PRSV), Watermelon mosaic virus (WMV), and Zucchini yellow mosaic virus (ZYMV). We collected 924 samples from 48 field plots. Out of these, almost 90% were infected by at least one of the viruses considered, usually CABYV, which was present in 83 and 66% of the melon and squash samples, respectively. In the case of melon, CYSDV, BPYV, and WMV followed CABYV in relative importance, with frequencies of around 20 to 30%, while in squash, CVYV and BPYY showed frequencies between 28 and 21%. The number of multiple infections was very high, 66 and 56% of the infected samples of melon and squash, respectively, being afflicted. CABYV was present in all multiple infections. The high incidence of CABYV in single and multiple infections suggests that this virus may well become an important threat for cucurbit crops in the region. Restriction fragment length polymorphism (RFLP) analysis revealed that CABYV isolates can be grouped into two genetic types, both of which seemed to be present during the 2003 epidemic episode, but only one of the types was found in 2004.

Inheritance of Resistance to Watermelon mosaic virus in Cucumis melo that Impairs Virus Accumulation, Symptom Expression, and Aphid Transmission

ABSTRACT The Cucumis melo accession TGR-1551 was found to be resistant to Watermelon mosaic virus (WMV, genus Potyvirus, family Potyviridae). The resistance resulted in a drastic and significant reduction of virus titer and infected plants were asymptomatic or exhibited mild disease symptoms. The same gene or closely linked genes restricted virus accumulation and ameliorated symptom expression. No effect was observed on virus accumulation in inoculated leaves, which suggested that the initial phases of infection were not affected. The resistance was effective against a range of WMV isolates from Spanish melon production areas. Using aphid inoculations, resistant plants showed a lower propensity for infection by WMV and for serving as virus sources for secondary spread once infected. Resistance was determined to be under recessive genetic control.